The experimental goals of the proposed studies are to determine the secondary structure of the 18S rRNA in both the free state and within the 40S subunit. Rabbit reticulocyte ribosomes are easily obtainable intact and in large quantities from anemic rabbits, and are actively engaged in the synthesis of predominantly one protein, hemoglobin. A complete nucleotide sequence of 18S rRNA has not yet been determined for any small eukaryotic ribosomal species. Seconday structure of both protein-free reticulocyte 18S rRNA, prepared by mild deproteinization, and 18S rRNA within the 40S subunit will be mapped using chemical modification, psoralen cross-linking, and nuclease susceptibility. Elucidation of the higher order structure of 18S rRNA in solution and within the ribosome will be essential for gaining insight not only into the functions of the ribosome such as in mRNA, t-RNA, and factor recognition and ribosomal subunit association, but also in such diverse problems as protein-nucleic acid recognition and evolutionary origin of the coding process.